Here are the findings from my first playthrough. I enjoyed the difficulty and found it fun, but some of the trinkets, not so much... I played without cheating or using the editor, up until the point I mention in my overview.
Switching to white ink because of spoilers... I hope this helps!
Level one:
Fine
Level two:
Overused Gameplay Mechanics - This was a bit of a difficulty spike. I didn't have any problems, but I died a lot more here than anywhere else in the level, mainly because of the three enemy formations without a checkpoint, and the need to do it twice again to get the trinket. Some may find this frustrating.
EDIT: At the end of this sequence, there's a terminal. If you use it, you're frozen in place in the path of an enemy and killed
Aim Well - I almost always end up slamming into the spikes in the next room here. Luckily, I know which side the checkpoint is on, so it's no big deal.
Dodge or Burn - At first, it seemed like the upper enemies should be thinned a little bit. Like maybe deleting one so there's a little hole you can aim for? Bouncing through the top part felt a bit like luck, and is a move I can't make repeatedly with any confidence.
Level three:
Going up - This sequence seems to rely on memorizing where the monsters are, and for the first few goes I was slamming into them without really having chance to react. Nothing wrong with this per se, as once you get the memorization done it's quite fun.
The Basement - Exiting left on the celing leads directly into spikes.
The Playroom - Exiting right on the celing, ditto. Though at least this time you'd have to be pretty stupid to do that!
Level four:
Positive Force - In the top lane you can exit right and appear buried inside a conveyor. Not game breaking stuff, but I would fix it if it were me.
Pressure Cooker - It's possible to exit up, and appear under the conveyor in the room above, only you're invisible because the room name blocks you. This would be quite confusing for a casual player, but isn't a huge problem.
Peircing the Sky - Sometimes, bouncing off of the top of the room will cause you to appear right on an enemy.. This caused several cheap deaths on my first run, because it's right where instinct lead me to try and bounce!
Popular Potpourri - I don't know, this just felt like a really weak room. The moving platform passing through the other platforms looked confusing and meh (use a bounding box to stop the platform!), and the gravity line along the top is suddenly spikes in the next room - I don't like that kind of discontinuity personally.
Pending Silence - Aesthetic gripe - the name is obscuring the floor that the spikes are on. It would look nicer with an extra layer of wall.
Phear - A lot of people have problems with 2-block openings surrounded by death. I enjoyed the challenge, but I've learned to limit that kind of thing mostly to optional challenges or hard mode versions.
Potential for Anything - I didn't need the leftmost platform to clear this room. I'm pointing it out in case you want to enforce your solution. It can be nice to be given some freedom, though.
Level five:
Small Big Difference - I found the top shock hole jump really difficult. In fact at first I thought it was impossible, but I got it in the end. Optional challenge, so no biggie.
Level six:
Away! - You can get stuck in the bottom left and right corner walls, and it sends Viridian crazy (I thought he'd just pop through the floor, but no...)
I Love Dew - You need to also plug the bottom right warp problem here... Also, maybe make the upper right wall 3 blocks thick like the other walls are, if you can? It's sorta throwing the visual balance off.
Level seven:
What's up? - Many will complain about your decision to have a Veni clone as part of the main course. I know I did this in my level, but I honestly found this harder than both mine and the original VVVVVV sequence. A couple of the rooms are also hard to 'read' in that you have zero time to respond to a bend because it's upon you right after a screen flick. That said, I did appreciate that you didn't go mental with this and the general course design, and I enjoyed the challenge, and beat it eventually. There's NO way I'm going back for that trinket, though! Seriously? All that and then a 2-block hole with spikes either side!? >_<
I Mean It - See the outcrop on the bottom left? I died so many times there because there's no time to respond to it. I know, muscle memory, but I find muscle memory works best when you've got a few microseconds to respond to visual clues. I'd push the spikes further up the screen if I were you. The rest of the course I thought was very reasonable and quite fun, but many will want an easier option. Perhaps consider widening it here and a couple of other places (such as the bottom right exit of At the Top?)
It was nice to see some screen wrapping in a Veni clone!
Level eight:
Very creative! And I didn't find any problems in any of the rooms. I did, however, find I could see the walls fairly easily, even though they were very dark. I think this will largely depend on the settings of the player's monitor. The guide lines in the background were a nice touch.
Eaten by a Grue - Found this quite challenging...
Level nine:
Arrghhhh!! I hate quizzes in games! I would have needed paper to do the algebra question, and I couldn't be bothered, so I just brute forced it.
The lying question got pretty much the same response from me... Maybe I'm being stupid and it's obvious, but I just guessed my way through.
The perfect square question also... Noticing a theme here? I'm not even sure, off the top of my head, what a perfect square is...
The 3D Maze was much more my cup of tea. Quite enjoyed that, as it fits in with the feeling of the game.
The memory test... This was quite a fun test, but I was unsure where I was supposed to start counting - at the very next room, or at the start of the teleporter sequence. You should clarify.
Level ten:
Level Up! - I'm not keen on the fact that you can exit left on the celing and appear on spikes. Given that this is near a checkpoint I'd fix it, but this is more a matter of taste. One spike in this room should be enough to provide continuity (and you might want to shunt the C over one as well, so respawners don't flail into the spike)
Masochist's Meal - Too many 2-block safety zones for my liking. Yes, it's possible, but it's not fun like the other challenges on the main path. I gave up on the trinket after getting very close.
Digits of Precision - >_< So not fun... There's simply no drama to these jumps. The difference between win and lose just isn't interesting. It's like throwing four dice until you get six on them all.
The Room Name Illusion - NO! JUST NO! I got through the last room after MANY tries, and you kill me with spikes deliberately hidden behind the room name?! That's cheap... This was the point that I decided to edit the level so I didn't have to go through that again.
Pearl's Gravitron - Wow, that was a really cool challenge. I felt pushed, and when I won, I felt I'd deserved it. This is a perfect 'functional end of level boss' screen for me... I just know that some players will find this too hard, though!
General notes:
Rooms I haven't mentioned are the ones I enjoyed and found no problems with. However, overall, while I appreciated the lack of tiling errors (horrible half-walls and the like) - I thought you could have made much more use of backing and other elements to 'prettify' the rooms. Subtle use of the different types of backing can really bring the aesthetics of a level to life.
There was also a spelling error in the second half of the game, but I can't remember where it was. I'll post when I run into it again.
The plot - I found that Viridian causing thousands of deaths made me distance myself from him. I didn't care about him because he was guilty and didn't seem to care. If he had simply unknowingly endangered the other dimension, it would have been more immersive, IMO. It's a small point, though.
I completed the game with 7 trinkets, and only got the normal ending.
Switching to white ink because of spoilers... I hope this helps!
Level one:
Fine
Level two:
Overused Gameplay Mechanics - This was a bit of a difficulty spike. I didn't have any problems, but I died a lot more here than anywhere else in the level, mainly because of the three enemy formations without a checkpoint, and the need to do it twice again to get the trinket. Some may find this frustrating.
EDIT: At the end of this sequence, there's a terminal. If you use it, you're frozen in place in the path of an enemy and killed

Aim Well - I almost always end up slamming into the spikes in the next room here. Luckily, I know which side the checkpoint is on, so it's no big deal.
Dodge or Burn - At first, it seemed like the upper enemies should be thinned a little bit. Like maybe deleting one so there's a little hole you can aim for? Bouncing through the top part felt a bit like luck, and is a move I can't make repeatedly with any confidence.
Level three:
Going up - This sequence seems to rely on memorizing where the monsters are, and for the first few goes I was slamming into them without really having chance to react. Nothing wrong with this per se, as once you get the memorization done it's quite fun.
The Basement - Exiting left on the celing leads directly into spikes.
The Playroom - Exiting right on the celing, ditto. Though at least this time you'd have to be pretty stupid to do that!
Level four:
Positive Force - In the top lane you can exit right and appear buried inside a conveyor. Not game breaking stuff, but I would fix it if it were me.
Pressure Cooker - It's possible to exit up, and appear under the conveyor in the room above, only you're invisible because the room name blocks you. This would be quite confusing for a casual player, but isn't a huge problem.
Peircing the Sky - Sometimes, bouncing off of the top of the room will cause you to appear right on an enemy.. This caused several cheap deaths on my first run, because it's right where instinct lead me to try and bounce!
Popular Potpourri - I don't know, this just felt like a really weak room. The moving platform passing through the other platforms looked confusing and meh (use a bounding box to stop the platform!), and the gravity line along the top is suddenly spikes in the next room - I don't like that kind of discontinuity personally.
Pending Silence - Aesthetic gripe - the name is obscuring the floor that the spikes are on. It would look nicer with an extra layer of wall.
Phear - A lot of people have problems with 2-block openings surrounded by death. I enjoyed the challenge, but I've learned to limit that kind of thing mostly to optional challenges or hard mode versions.
Potential for Anything - I didn't need the leftmost platform to clear this room. I'm pointing it out in case you want to enforce your solution. It can be nice to be given some freedom, though.
Level five:
Small Big Difference - I found the top shock hole jump really difficult. In fact at first I thought it was impossible, but I got it in the end. Optional challenge, so no biggie.
Level six:
Away! - You can get stuck in the bottom left and right corner walls, and it sends Viridian crazy (I thought he'd just pop through the floor, but no...)
I Love Dew - You need to also plug the bottom right warp problem here... Also, maybe make the upper right wall 3 blocks thick like the other walls are, if you can? It's sorta throwing the visual balance off.
Level seven:
What's up? - Many will complain about your decision to have a Veni clone as part of the main course. I know I did this in my level, but I honestly found this harder than both mine and the original VVVVVV sequence. A couple of the rooms are also hard to 'read' in that you have zero time to respond to a bend because it's upon you right after a screen flick. That said, I did appreciate that you didn't go mental with this and the general course design, and I enjoyed the challenge, and beat it eventually. There's NO way I'm going back for that trinket, though! Seriously? All that and then a 2-block hole with spikes either side!? >_<
I Mean It - See the outcrop on the bottom left? I died so many times there because there's no time to respond to it. I know, muscle memory, but I find muscle memory works best when you've got a few microseconds to respond to visual clues. I'd push the spikes further up the screen if I were you. The rest of the course I thought was very reasonable and quite fun, but many will want an easier option. Perhaps consider widening it here and a couple of other places (such as the bottom right exit of At the Top?)
It was nice to see some screen wrapping in a Veni clone!
Level eight:
Very creative! And I didn't find any problems in any of the rooms. I did, however, find I could see the walls fairly easily, even though they were very dark. I think this will largely depend on the settings of the player's monitor. The guide lines in the background were a nice touch.
Eaten by a Grue - Found this quite challenging...

Level nine:
Arrghhhh!! I hate quizzes in games! I would have needed paper to do the algebra question, and I couldn't be bothered, so I just brute forced it.
The lying question got pretty much the same response from me... Maybe I'm being stupid and it's obvious, but I just guessed my way through.
The perfect square question also... Noticing a theme here? I'm not even sure, off the top of my head, what a perfect square is...
The 3D Maze was much more my cup of tea. Quite enjoyed that, as it fits in with the feeling of the game.
The memory test... This was quite a fun test, but I was unsure where I was supposed to start counting - at the very next room, or at the start of the teleporter sequence. You should clarify.
Level ten:
Level Up! - I'm not keen on the fact that you can exit left on the celing and appear on spikes. Given that this is near a checkpoint I'd fix it, but this is more a matter of taste. One spike in this room should be enough to provide continuity (and you might want to shunt the C over one as well, so respawners don't flail into the spike)
Masochist's Meal - Too many 2-block safety zones for my liking. Yes, it's possible, but it's not fun like the other challenges on the main path. I gave up on the trinket after getting very close.
Digits of Precision - >_< So not fun... There's simply no drama to these jumps. The difference between win and lose just isn't interesting. It's like throwing four dice until you get six on them all.
The Room Name Illusion - NO! JUST NO! I got through the last room after MANY tries, and you kill me with spikes deliberately hidden behind the room name?! That's cheap... This was the point that I decided to edit the level so I didn't have to go through that again.
Pearl's Gravitron - Wow, that was a really cool challenge. I felt pushed, and when I won, I felt I'd deserved it. This is a perfect 'functional end of level boss' screen for me... I just know that some players will find this too hard, though!
General notes:
Rooms I haven't mentioned are the ones I enjoyed and found no problems with. However, overall, while I appreciated the lack of tiling errors (horrible half-walls and the like) - I thought you could have made much more use of backing and other elements to 'prettify' the rooms. Subtle use of the different types of backing can really bring the aesthetics of a level to life.
There was also a spelling error in the second half of the game, but I can't remember where it was. I'll post when I run into it again.
The plot - I found that Viridian causing thousands of deaths made me distance myself from him. I didn't care about him because he was guilty and didn't seem to care. If he had simply unknowingly endangered the other dimension, it would have been more immersive, IMO. It's a small point, though.
I completed the game with 7 trinkets, and only got the normal ending.